The invention relates to a method for oxygen production by low-temperature separation of air with variable energy consumption in a distillation column system having a high-pressure column, a low-pressure column as well as a main condenser and a side condenser which are both in the form of condenser-evaporators, wherein in the method                atmospheric air is compressed to a total air pressure in a main air compressor, cooled in a main heat exchanger and fed at least in part to the high-pressure column,        in the main condenser, gaseous nitrogen from the high-pressure column is at least partially liquefied,        at least a portion of the liquid nitrogen generated in the main condenser is used as reflux in at least one of the columns of the distillation column system,        a first liquid oxygen stream from the bottom of the low-pressure column is introduced into the side condenser and is at least partially evaporated therein in indirect heat exchange with at least a portion of the compressed and cooled feed air,        at least a portion of the evaporated first liquid oxygen stream is obtained as a gaseous oxygen product,        in a first operating mode with higher energy consumption        a first amount of the first liquid oxygen stream from the bottom of the low-pressure column is introduced into the side condenser and        a first amount of air is compressed in the main air compressor to a first outlet pressure,in a second operating mode        a second amount of air, which is smaller than the first amount of air, is compressed in the main air compressor,        a second amount of the first liquid oxygen stream from the bottom of the low-pressure column, which is smaller than the first amount, is introduced into the side condenser, and        a second liquid oxygen stream is fed to the side condenser in addition to the first liquid oxygen stream. The method and the device of the invention are suitable in particular for producing gaseous impure oxygen. “Impure oxygen” is here understood as being a product having a purity of less than 98 mol. %.        
Methods and devices for the low-temperature separation of air are known, for example, from Hausen/Linde, Tieftemperaturtechnik, 2nd Edition, 1985, Chapter 4 (pages 281 to 337).
The distillation column system can be in the form of a two-column system (for example in the form of a conventional Linde double column system) or alternatively in the form of a system having three or more columns. In addition to the columns for nitrogen-oxygen separation, it can have further devices for producing highly pure products and/or other air components, in particular noble gases, for example for argon production and/or krypton-xenon production.
The “low-pressure column” is here understood as being a uniform distillation region in which the pressure is constant apart from the natural pressure loss at the material exchange elements. This distillation region can be arranged in one or more containers.
The “main heat exchanger” serves to cool feed air in indirect heat exchange with return streams from the distillation column system. It can be formed of a single heat exchanger section or of a plurality of heat exchanger sections connected in parallel and/or in series, for example of one or more plate heat exchanger blocks.
“Condenser-evaporator” refers to a heat exchanger in which a first, condensing fluid stream comes into indirect heat exchange with a second, evaporating fluid stream. Each condenser-evaporator has a liquefaction space and an evaporation space, which consist of liquefaction passages and evaporation passages, respectively. In the liquefaction space, the condensation (liquefaction) of a first fluid stream is carried out; in the evaporation space, the evaporation of a second fluid stream is carried out. The evaporation and liquefaction spaces are formed by groups of passages which are in heat exchange relationship with one another.
A “side condenser” is to be understood as being a condenser-evaporator which is designed almost exclusively for the indirect transfer of latent heat from a condensing process stream evaporation to an evaporating process stream against a second, condensing process stream and is not or substantially not suitable for the transfer of sensible heat. It is formed by a heat exchanger which is separate from other heat exchangers, in particular a main heat exchanger or a supercooling countercurrent heat exchanger, both of which generally serve solely or predominantly for the heat exchange of purely gaseous streams.
“Amounts” of streams here refer to the mass flow rate, measured, for example, in Nm3/h.
In this application, process parameters such as mass streams or pressures are repeatedly described which are “smaller” or “larger” in one operating mode than in another operating mode. This means purposive changes of the corresponding parameter by regulating and/or control devices and not natural variations within a steady-state operating state. These purposive changes can be effected directly by adjusting the parameter itself or indirectly by adjusting other parameters which influence the parameter to be changed. In particular, a parameter is “larger” or “smaller” when the difference between the mean values of the parameter in the different operating modes is more than 2%, in particular more than 5%, in particular more than 10%.
The “first liquid oxygen stream” is the mass stream of liquid oxygen that is removed from the low-pressure column and introduced into the evaporation space of the side condenser. It can be the total amount of the liquid oxygen removed from the low-pressure column. The first liquid oxygen stream can, however, also consist of only a portion of the liquid oxygen removed from the low-pressure column, for example when a liquid oxygen product is additionally obtained from the low-pressure column and fed to a liquid tank. If a liquid oxygen product is drawn from the evaporation space of the side condenser, it is generally formed by a portion of the “first liquid oxygen stream”. Conversely, liquid oxygen additional to the first liquid oxygen stream can in principle be fed to the side condenser.
The “second liquid oxygen stream” represents the difference between the total amount of liquid oxygen introduced into the evaporation space of the side condenser and the first liquid oxygen stream. The second liquid oxygen stream is removed from a liquid tank, for example. The liquid tank can be filled solely from an external source, solely with liquid oxygen from the low-pressure column (as in Springmann, see below), or partly with external liquid oxygen and partly with liquid oxygen formed in the distillation column system, in particular in the low-pressure column or in the evaporation space of the side condenser.
A method of the type mentioned at the beginning and a corresponding device are known from Springmann, “Energieeinsparung”, Linde-Symposium “Luftzerlegungs-anlagen”, 4th seminar of Linde AG of Oct. 15-17, 1980, Article H. An alternative reservoir process with two liquid tanks is shown therein. However, that process is carried out not with a constant throughput through the distillation column system with a varying product amount, but with varying operation in dependence on varying energy costs. When the energy price is low, oxygen is produced for stock and stored in a liquid tank. When the energy price is high, the amount of air is reduced and a portion of the oxygen product is removed from the stock. The separative work performed on the stored oxygen is thus available for energy storage. According to this teaching, in times of cheap energy the liquid air is replaced with liquid oxygen in the plant, that is to say liquid oxygen is fed into the tank and the equivalent amount of liquid air is fed from the corresponding tank into the distillation column system. Conversely, in times of high electricity prices, liquid oxygen from the tank is fed into the system and liquid air is stored. Accordingly, virtually only the stored oxygen molecules are available for energy storage; in times of high electricity prices, the main air compressor has to deliver correspondingly less separation air.
The object underlying the invention is to improve the efficiency of such a method in terms of energy storage.